JUP and arthritic joint disease: Serum samples were obtained from patients with RA (n = 155), PD subjects (n = 48), and HCs (n = 35). The profile of antibody response to gingipain RgpA-specific domains, a cysteine protease produced by Pg, was determined in all included patients’ sera, and the potential protective effects of RgpA domains in an experimental arthritis model were also tested.